if you want to be creative with the solution
You could create an "ItemAdding" event receiver to check to see if the user is from a particular sharepoint group and display an appropriate message accordingly.
Here is an article that should give you a general idea on how to go about it.
http://sharepointcoding.wordpress.com/2008/08/12/creating-a-simple-event-handler-for-sharepoint-part-1/